I have 2 different VPS, both KVM-based, sysinfo is identical on both, and I have identical website on both.When using proxy_fcgi_event one of the servers has no issues, but the other server have directory permission errors. On that server, if I set certain directory permissions to 777 there are no issues.

When I swithch both servers to php-fpm_event mode then domain error log shows [FastCGI: neither POLLIN nor POLLOUT...] [mydomain.com] at regular intervals on both servers.

With this error, do you thin that the response time will slow down significatly if there are many logged in to the domain at the same time ?

If you use fastcgi_event or proxy_fcgid as PHP type in Webserver config then, as Mustafa have explained, you will most likely experience some file write issues, especially for files and directories used for various types of cache files. For example the cache directory located in the file sturcture below your domain root. This can be fixed as Mustafa has explained, by changing ownership to apache:apache for user and group for those directories & files concerned, by using the change ownership tool available in Kloxo-MR File Manager.